#include <sys/prctl.h>
#include <errno.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <syscall.h>

/*
 * Sets the process title to the specified title. Note that this may fail if
 * the kernel doesn't support PR_SET_MM_MAP (kernels <3.18).
 */
int setproctitle(char *title)
{
	FILE *f = NULL;
	int i, fd, len;
	char *buf_ptr, *tmp_proctitle;
	char buf[4096];
	int ret = 0;
	ssize_t bytes_read = 0;
	static char *proctitle = NULL;

	/*
	 * We don't really need to know all of this stuff, but unfortunately
	 * PR_SET_MM_MAP requires us to set it all at once, so we have to
	 * figure it out anyway.
	 */
	unsigned long start_data, end_data, start_brk, start_code, end_code,
		start_stack, arg_start, arg_end, env_start, env_end, brk_val;
	struct prctl_mm_map prctl_map;

	f = fopen("/proc/self/stat", "r");
	if (!f) {
		fprintf(stderr, "fopen(stat): '%m' (%d)\n", errno);
		return -1;
	}

	fd = fileno(f);
	if (fd < 0) {
		fprintf(stderr, "fileno(%p): '%m' (%d)\n", f, errno);
		fclose(f);
		return -1;
	}

	bytes_read = read(fd, buf, sizeof(buf) - 1);
	if (bytes_read <= 0) {
		fprintf(stderr, "read(): '%m' (%d)\n", errno);
		fclose(f);
		return -1;
	}

	buf[bytes_read] = '\0';

	/* Skip the first 25 fields, column 26-28 are start_code, end_code,
	 * and start_stack */
	buf_ptr = strchr(buf, ' ');
	for (i = 0; i < 24; i++) {
		if (!buf_ptr) {
			fclose(f);
			return -1;
		}
		buf_ptr = strchr(buf_ptr + 1, ' ');
	}
	if (!buf_ptr) {
		fclose(f);
		return -1;
	}

	i = sscanf(buf_ptr, "%lu %lu %lu", &start_code, &end_code, &start_stack);
	if (i != 3) {
		fclose(f);
		return -1;
	}

	/* Skip the next 19 fields, column 45-51 are start_data to arg_end */
	for (i = 0; i < 19; i++) {
		if (!buf_ptr) {
			fclose(f);
			return -1;
		}
		buf_ptr = strchr(buf_ptr + 1, ' ');
	}

	if (!buf_ptr) {
		fclose(f);
		return -1;
	}

	i = sscanf(buf_ptr, "%lu %lu %lu %*u %*u %lu %lu", &start_data,
		   &end_data, &start_brk, &env_start, &env_end);
	if (i != 5) {
		fclose(f);
		return -1;
	}

	/* Include the null byte here, because in the calculations below we
	 * want to have room for it. */
	len = strlen(title) + 1;

	tmp_proctitle = realloc(proctitle, len);
	if (!tmp_proctitle) {
		fclose(f);
		return -1;
	}

	proctitle = tmp_proctitle;

	arg_start = (unsigned long)proctitle;
	arg_end = arg_start + len;

	brk_val = syscall(__NR_brk, 0);

	prctl_map = (struct prctl_mm_map){
		.start_code = start_code,
		.end_code = end_code,
		.start_stack = start_stack,
		.start_data = start_data,
		.end_data = end_data,
		.start_brk = start_brk,
		.brk = brk_val,
		.arg_start = arg_start,
		.arg_end = arg_end,
		.env_start = env_start,
		.env_end = env_end,
		.auxv = NULL,
		.auxv_size = 0,
		.exe_fd = -1,
	};

	ret = prctl(PR_SET_MM, PR_SET_MM_MAP, &prctl_map,
		    sizeof(prctl_map), 0);
	if (ret == 0)
		(void)strncpy((char *)arg_start, title, len);
	else
		fprintf(stderr, "Failed to set cmdline\n");

	fclose(f);

	return ret;
}

int main(int argc, char *argv[])
{
	// ...
	// show pid to find the right process
	pid_t pid = getpid();
	printf("pid = %d\n", pid);

	if (argv[1]) {
		setproctitle(argv[1]);
		// ...
	}

	sleep(1000);
	return 0;
}
